1. Field of the Invention
The present invention relates to an ATM (Asynchronous Transfer Mode) system for use in ATM communication to conduct rate control of cells on an ATM network.
2. Description of the Related Art
An ATM system includes an interface unit having large-scale input/output cell buffers and a switch unit. The switch unit has a function of switching an input cell and includes a small-scale output buffer. Because the switch unit is required to conduct processing at a speed highest in the devices of the ATM system, arrangement of a large-scale buffer with a complicated control function in the unit is very difficult. On the other hand, the interface unit has large-scale input/output buffers which are hard to be provided in the switch unit and an output scheduling function for conducting various kinds of control related to cell output processing. Kinds of control executed as the output scheduling function are, for example, priority control taking quality of service required corresponding to a connection into consideration, control for adjusting a cell flow rate according to a band defined in advance and BP (Back Pressure) control for stopping output of a cell to a specific line on a QoS (Quality of Service) class basis according to a state of cell accumulation in an output buffer of the switch unit or an output buffer of the interface unit.
ATM Forum defines control for clearly expressing existence/non-existence of congestion and ER (Explicit Rate) control for expressing a transmission-authorized rate as End-to-End rate control for ABR (Available Bit Rate) traffic service which is one of QoS classes. It is well known that ER control is effective in terms of transient response after the occurrence of congestion and equality of throughput. Several methods of calculating an ER for use in ER control are presented for reference at ATM Forum, one of which is the EPRCA (Enhanced Proportional Rate Control Algorithm) method using, as ER, a pseudo mean value calculated from CCR (Current Cell Rate) in RM (Resource Management) cells and the ERICA (Explicit Rate Indication for Congestion Avoidance) method using, as ER, a value obtained by dividing an effective band by the number of active VCs (Virtual Channels). Structure of an ATM system including an EPRCA-method-based ER arithmetic means is shown in FIG. 6. With reference to FIG. 6, in each of a plurality of interface units 610 connected to a switch unit 620, an ER arithmetic unit 611 for performing ER arithmetic based on the EPRCA method is provided.
The above-described ER calculation methods presented in ATM Forum, however, might invite deterioration of performance due to unstable operation in actual operating environments because of the use of CCR information which is not always accurate and the number of active VCs which is difficult to calculate at a switch unit and also because of large dependency on information such as an ABR background band.
Another proposed ER calculation method is a DPDC (Dual Proportional Differential Controller) method by which ER involving a little change of the amount of accumulation in a buffer with time is derived by feedback control. This method is disclosed, for example, in the literature "Large Scale ATM Switch with Multiple QoS Guarantee" (Shinohara, Suzuki, Fan, Ramamurthy, The Institute of Electronics Information and Communication Engineers, Technical Report of IEICE, 1996-8, pp. 73-78). Structure of an ATM system including a DPDC-method-based ER arithmetic means is shown in FIG. 7. Also in a case of the DPDC method, when each interface unit 710 independently conducts control as illustrated in FIG. 7, problems occur that a throughput for the same destination at each interface unit 710 is smoothed due to BP control according to a state of an output buffer 721 of a switch unit 720 or an output buffer 712 of the interface unit 710 and that an ER value for the same destination varies with each interface unit 710 because each interface unit 710 executes ER calculation independently. In other words, when the number of connections for the same destination at each interface unit 710 is imbalanced, equality of throughput among connections can not be maintained.
ATM Forum also defines UBR (Unspecified Bit Rate) traffic service as one of QoS classes. For UBR, there is neither clear definition of quality nor such definition of terminal/Internet feedback control as for ABR. However, with respect to Internet traffic as an object of UBR service, there is a strong demand for improvement in equality. Relationship in connection between an interface unit and a switch unit in a conventional ATM system is shown in detail in FIG. 8. In the conventional ATM system illustrated in FIG. 8, with the order of output priority of UBR set to be lower than those of other classes and with no specific output rate set, a cell flow rate is controlled only by BP control derived from an output buffer.
The traffic control in the UBR, as well as in ABR, however, has a drawback that because a throughput for the same destination at each interface unit 810 is smoothed due to BP control according to a state of an output buffer 821 of a switch unit 820 or an output buffer 812 of the interface unit 810, when the number of connections for the same destination at each interface unit 810 is imbalanced, equality of throughput among connections can not be maintained. Although this shortcoming should be eliminated with respect not only to UBR but also to every QoS class, this is acute in UBR which is set to have a lower priority order.
In order to maintain equality of throughput among connections, an internal rate control method is proposed in which ER control is employed also for other QoS classes (UBR in particular) than ABR as well as ABR and ER control is conducted in cooperation among the respective interface units. Realization of this method requires provision of a means for communicating information between the respective interface units. However, since such a communication means is not defined in ATM Forum and with respect to UBR, in particular, no RM cell is defined, the rate control is not realizable.
As described in the foregoing, conventional ATM systems have a drawback that preventing deterioration of performance results in that equality of throughput among connections can not be maintained depending on a calculation method of an ER for use in ER control. The reasons are that a throughput for the same destination at each interface unit is smoothed due to BP control derived from an output buffer and that because when ER control is employed for other QoS classes than ABR, each interface unit independently conducts ER calculation, so that an ER value for the same destination varies with each interface unit.
In addition, when ER control is employed for other QoS classes than ABR, ER control in cooperation with the respective interfaces is not realizable in practice because no appropriate means exists for conducting information communication necessary for the cooperation among the interface units.